SI-8428 Fix regression in iterator concatenation

Regressed in e3ddb2d7, which introduced `ConcatIterator` to
avoid super-linear runtime on chains of concatenated iterators.

`ConcatIterator` maintains a queue of thunks for the remaining
iterators. Both `next` and `hasNext` delegate to `advance`, which
evaluates the thunks and discards any empty iterators from the
start of the queue. The first non-empty iterator is stored in
the var `current`.

It also overrides `++`, and creates a new `ConcatIterator` with
the given `that` as an extra element in the queue. However, it
failed to copy `current` across, which led to data loss.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/library/scala/collection/Iterator.scala b/src/library/scala/collection/Iterator.scala
index 01a0aa3b51..3d379bfa95 100644
--- a/src/library/scala/collection/Iterator.scala
+++ b/src/library/scala/collection/Iterator.scala
@@ -185,7 +185,7 @@ object Iterator {
     def next()  = if (hasNext) current.next else Iterator.empty.next
 
     override def ++[B >: A](that: => GenTraversableOnce[B]): Iterator[B] =
-      new ConcatIterator(queue :+ (() => that.toIterator))
+      new ConcatIterator((() => current) +: (queue :+ (() => that.toIterator)))
   }
 
   private[scala] final class JoinIterator[+A](lhs: Iterator[A], that: => GenTraversableOnce[A]) extends Iterator[A] {